Method of manufacturing a semiconductor device and a semiconductor device

ABSTRACT

In a method, a fin structure, in which first semiconductor layers and second semiconductor layers are alternately stacked, is formed. A sacrificial gate structure is formed over the fin structure. The first semiconductor layers are etched at a source/drain region of the fin structure, which is not covered by the sacrificial gate structure, thereby forming a first source/drain space in which the second semiconductor layers are exposed. A dielectric layer is formed at the first source/drain space, thereby covering the exposed second semiconductor layers. The dielectric layer and part of the second semiconductor layers are etched, thereby forming a second source/drain space. A source/drain epitaxial layer is formed in the second source/drain space. At least one of the second semiconductor layers is in contact with the source/drain epitaxial layer, and at least one of the second semiconductor layers is separated from the source/drain epitaxial layer.

TECHNICAL FIELD

The disclosure relates to method of manufacturing semiconductorintegrated circuits, and more particularly to method of manufacturingsemiconductor devices including fin field effect transistors (FinFETs)and/or gate-all-around (GAA) FETs, and semiconductor devices.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as amulti-gate field effect transistor (FET), including a fin FET (FinFET)and a gate-all-around (GAA) FET. In a Fin FET, a gate electrode isadjacent to three side surfaces of a channel region with a gatedielectric layer interposed therebetween. Because the gate structuresurrounds (wraps) the fin on three surfaces, the transistor essentiallyhas three gates controlling the current through the fin or channelregion. Unfortunately, the fourth side, the bottom part of the channelis far away from the gate electrode and thus is not under close gatecontrol. In contrast, in a GAA FET, all side surfaces of the channelregion are surrounded by the gate electrode, which allows for fullerdepletion in the channel region and results in less short-channeleffects due to steeper sub-threshold current swing (SS) and smallerdrain induced barrier lowering (DIBL). As transistor dimensions arecontinually scaled down to sub 10-15 nm technology nodes, furtherimprovements of the GAA FET are required.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 shows a cross sectional view of a semiconductor FET deviceaccording to an embodiment of the present disclosure.

FIG. 2 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 3 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 4 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 5 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 6 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 7 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 8 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 9 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 10 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 11 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIGS. 12A, 12B, 12C and 12D show one of the various stages ofmanufacturing a semiconductor FET device according to an embodiment ofthe present disclosure.

FIGS. 13A, 13B, 13C, 13D, 13E and 13F show one of the various stages ofmanufacturing a semiconductor FET device according to an embodiment ofthe present disclosure.

FIG. 14 shows various etching operations for manufacturing structuresshown in FIGS. 13A-13F.

FIG. 15 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 16 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 17 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 18 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 19 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIG. 20 shows one of the various stages of manufacturing a semiconductorFET device according to an embodiment of the present disclosure.

FIGS. 21A, 21B, 21C and 21D show one of the various stages ofmanufacturing a semiconductor FET device according to an embodiment ofthe present disclosure.

FIG. 22 shows a cross sectional view of a semiconductor FET deviceaccording to another embodiment of the present disclosure.

FIG. 23 shows one of the various stages of manufacturing a semiconductorFET device according to another embodiment of the present disclosure.

FIG. 24 shows one of the various stages of manufacturing a semiconductorFET device according to another embodiment of the present disclosure.

FIGS. 25A, 25B, 25C, 25D, 25E and 25F show one of the various stages ofmanufacturing a semiconductor FET device according to another embodimentof the present disclosure.

FIG. 26 shows a NOR circuit using semiconductor FET devices according toan embodiment of the present disclosure.

FIG. 27 shows a NAND circuit using semiconductor FET devices accordingto an embodiment of the present disclosure.

FIG. 28A shows a cross sectional view of a semiconductor FET device andFIG. 28B shows a layout using the semiconductor FET device according toan embodiment of the present disclosure.

FIG. 29A shows a static random access memory (SRAM) cell diagram andFIG. 29B shows various transistor properties for the SRAM cell accordingto an embodiment of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A, B, C, A and B, A and C, B and C, or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

In a GAA FET, a driving current can be adjusted by changing dimensions(e.g., a cross sectional area) of each semiconductor wires constitutingchannel regions and/or by changing the number of semiconductor wiresvertically arranged over a substrate. However, it is generally difficultto manufacture GAA FETs with different numbers of semiconductor wireswithin one semiconductor chip.

In the present disclosure, the driving current of GAA FETs is modulatedby changing the number of semiconductor wires electrically connected toa source/drain epitaxial layer. In this disclosure, a source/drainrefers to a source and/or a drain. It is noted that in the presentdisclosure, a source and a drain are interchangeably used and thestructures thereof are substantially the same, unless otherwisedescribed.

FIG. 1 shows a cross sectional view of a semiconductor FET deviceaccording to an embodiment of the present disclosure.

As shown in FIG. 1, a first GAA FET Q1 and a second GAA FET Q2 aredisposed over a substrate 10. In each of the first and second GAA FETs,semiconductor wires 25 are provided over the semiconductor substrate 10,and vertically arranged along the Z direction (the normal direction ofthe principal surface of the substrate 10). In some embodiments, thesubstrate 10 includes a single crystalline semiconductor layer on atleast it surface portion. The substrate 10 may comprise a singlecrystalline semiconductor material such as, but not limited to Si, Ge,SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb and InP. Incertain embodiments, the substrate 10 is made of crystalline Si.

The substrate 10 may include in its surface region, one or more bufferlayers (not shown). The buffer layers can serve to gradually change thelattice constant from that of the substrate to that of the source/drainregions. The buffer layers may be formed from epitaxially grown singlecrystalline semiconductor materials such as, but not limited to Si, Ge,GeSn, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb, GaN,GaP, and InP. In a particular embodiment, the substrate 10 comprisessilicon germanium (SiGe) buffer layers epitaxially grown on the siliconsubstrate 10. The germanium concentration of the SiGe buffer layers mayincrease from 30 atomic % germanium for the bottom-most buffer layer to70 atomic % germanium for the top-most buffer layer.

In each of the first and second GAA FETs, the semiconductor wires 25,which are channel layers, are disposed over the substrate 10. In someembodiments, the semiconductor wires 25 are disposed over a finstructure (not shown) protruding from the substrate 10. Each of thechannel layers 25 is wrapped around by a gate dielectric layer 53 and agate electrode layer 58. In some embodiments, the gate dielectric layer53 includes an interfacial layer 52 and a high-k dielectric layer 54.The gate structure includes the gate dielectric layer 53, the gateelectrode layer 58 and sidewall spacers 32. Although FIG. 1 shows foursemiconductor wires 25, the number of the semiconductor wires 25 is notlimited to four, and may be as small as one or more than four and may beup to fifteen (15).

In certain embodiments of the present disclosure, one or more workfunction adjustment layers 56 are interposed between the gate dielectriclayer 53 and the gate electrode layer 58.

In each of the first and second GAA FETs, a source/drain epitaxial layer40 is disposed over the substrate 10. The source/drain epitaxial layer40 is in direct contact with the channel layer 25, and is separated by adielectric layer 35 as inner spacers and the gate dielectric layer 53from the gate electrode layer 58. The dielectric layer 35 is made of alow-k (low dielectric constant lower than the dielectric constant ofSiO₂) material. The low-k material includes SiOC, SiOCN, organicmaterial or porous material, or any other suitable material.

An interlayer dielectric (ILD) layer 36 is disposed over the S/Depitaxial layer 40 and a conductive contact layer 60 is disposed on theS/D epitaxial layer 40 and a conductive plug 65 passing though the ILDlayer 36 is disposed over the conductive contact layer 60. Theconductive contact layer 60 includes one or more layers of conductivematerial. In some embodiments, the conductive contact layer 60 includesa silicide layer, such as WSi, NiSi, TiSi or CoSi or other suitablesilicide material.

The first GAA FET Q1 and the second GAA FET Q2 have substantially thesame structure except for the source/drain regions. As shown in FIG. 1,the source/drain epitaxial layer 40 of the first GAA FET Q1 is inphysical and electrical contact with all of the semiconductor wires 25,while the source/drain epitaxial layer 40 of the second GAA FET Q2 is inphysical and electrical contact with only some of the semiconductorwires 25. In some embodiments, as shown in FIG. 1, the source/drainepitaxial layer 40 of the second GAA FET Q2 is in physical andelectrical contact with two of the four semiconductor wires 25. In otherwords, at least one of the semiconductor wires 25 of the second GAA FETQ2 is electrically separated from the source/drain epitaxial layer 40disposed thereabove, by the dielectric layer 35.

In certain embodiments, at least one of the semiconductor wires 25 ofthe first GAA FET Q1 is electrically separated from the source/drainepitaxial layer 40 disposed thereabove, by the dielectric layer 35. Insuch a case, the number of the semiconductor wires 25 contacting thesource/drain epitaxial layer 40 in the first GAA FET Q1 is the same asor different from the number of the semiconductor wires 25 contactingthe source/drain epitaxial layer 40 in the second GAA FET Q2.

As shown in FIG. 1, when one or more of the semiconductor wires 25 arenot in contact with the source/drain epitaxial layer 40 in thesource/drain region, the one or more of the semiconductor wires 25 notin contact with the source/drain epitaxial layer 40 are wrapped with thegate dielectric layer 53 and the gate electrode layer 58.

One or more of the semiconductor wires 25 are electrically separatedfrom the source/drain epitaxial layer 40, they are located closer to thesubstrate 10 than the remaining semiconductor wires 25 contacting thesource/drain epitaxial layer.

In some embodiments, two or more of the semiconductor wires 25 areelectrically separated from the source/drain epitaxial layer 40. Inother embodiments, only one of the semiconductor wires 25 is in contactwith the source/drain epitaxial layer 40.

By adjusting the number of the semiconductor wires 25 contacting thesource/drain epitaxial layer 40, it is possible to adjust a drivingcurrent of a GAA FET. When the number is larger, the driving currentincreases, and when the number is smaller, an input capacitance can bereduced and an operational speed can be increased.

As set forth above, FIG. 1 shows GAA FETs. However, the structures donot necessarily function as a transistor. In some embodiments, thestructures shown in FIG. 1 can function as resistors. In such a case,the gate electrode is coupled to a fixed potential, such as Vdd (e.g.,positive power supply), ½Vdd or Vss (e.g., the ground). The resistancevalue can be adjusted by adjusting the number of the semiconductor wires25 contacting the source/drain epitaxial layer 40. For example, assumingthat the resistance value between the source and drain is 4R when thenumber of the semiconductor wires 25 contacting the source/drainepitaxial layer 40 is one, the resistance values 2R, 1.25R and R can beobtained when the number of the semiconductor wires 25 contacting thesource/drain epitaxial layer 40 is two, three and four, respectively.

FIGS. 2-21D show various stages of manufacturing a semiconductor FETdevice according to an embodiment of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIGS. 2-21D, and some of the operationsdescribed below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIG. 1 may be employed in the embodiment of FIGS. 2-21D, anddetailed explanation thereof may be omitted. General methods ofmanufacturing a GAA FET can be found in U.S. patent application Ser. No.15/157,139, application Ser. No. 15/064,402 and/or application Ser. No.15/098,073, the entire contents of each of which are incorporated hereinby reference.

As shown in FIG. 2, impurity ions (dopants) 12 are implanted into asilicon substrate 10 to form a well region. The ion implantation isperformed to prevent a punch-through effect. The substrate 10 mayinclude various regions that have been suitably doped with impurities(e.g., p-type or n-type conductivity). The dopants 12 are, for exampleboron (BF₂) for an n-type Fin FET and phosphorus for a p-type Fin FET.

Then, as shown in FIG. 3, stacked semiconductor layers are formed overthe substrate 10. The stacked semiconductor layers include firstsemiconductor layers 20 and second semiconductor layers 25. Further, amask layer 16 is formed over the stacked layers.

The first semiconductor layers 20 and the second semiconductor layers 25are made of materials having different lattice constants, and mayinclude one or more layers of Si, Ge, SiGe, GaAs, InSb, GaP, GaSb,InAlAs, InGaAs, GaSbP, GaAsSb or InP.

In some embodiments, the first semiconductor layers 20 and the secondsemiconductor layers 25 are made of Si, a Si compound, SiGe, Ge or a Gecompound. In one embodiment, the first semiconductor layers 20 areSi_(1-x)Ge_(x), where x is more than about 0.3, or Ge (x=1.0) and thesecond semiconductor layers 25 are Si or Si_(1-y)Ge_(y), where y is lessthan about 0.4, and x>y. In this disclosure, an “M” compound” or an “Mbased compound” means the majority of the compound is M.

In another embodiment, the second semiconductor layers 25 areSi_(1-y)Ge_(y), where y is more than about 0.3, or Ge, and the firstsemiconductor layers 20 are Si or Si_(1-x)Ge_(x), where x is less thanabout 0.4, and x<y. In yet other embodiments, the first semiconductorlayer 20 is made of Si_(1-x)Ge_(x), where x is in a range from about 0.3to about 0.8, and the second semiconductor layer 25 is made ofSi_(1-y)Ge_(y), where y is in a range from about 0.1 to about 0.4.

In FIG. 3, four layers of the first semiconductor layer 20 and fourlayers of the second semiconductor layer 25 are disposed. However, thenumber of the layers are not limited to four, and may be as small as 1(each layer) and in some embodiments, 2-10 layers of each of the firstand second semiconductor layers are formed. By adjusting the numbers ofthe stacked layers, a driving current of the GAA FET device can beadjusted.

The first semiconductor layers 20 and the second semiconductor layers 25are epitaxially formed over the substrate 10. The thickness of the firstsemiconductor layers 20 may be equal to or greater than that of thesecond semiconductor layers 25, and is in a range from about 2 nm toabout 20 nm in some embodiments, and is in a range from about 5 nm toabout 15 nm in other embodiments. The thickness of the secondsemiconductor layers 25 is in a range from about 2 nm to about 20 nm insome embodiments, and is in a range from about 5 nm to about 15 nm inother embodiments. The thickness of each of the first semiconductorlayers 20 may be the same, or may vary.

In some embodiments, the bottom first semiconductor layer (the closestlayer to the substrate 10) is thicker than the remaining firstsemiconductor layers. The thickness of the bottom first semiconductorlayer is in a range from about 10 nm to about 50 nm in some embodiments,or is in a range from 20 nm to 40 nm in other embodiments.

In some embodiments, the mask layer 16 includes a first mask layer 16Aand a second mask layer 16B. The first mask layer 16A is a pad oxidelayer made of a silicon oxide, which can be formed by a thermaloxidation. The second mask layer 16B is made of a silicon nitride (SiN),which is formed by chemical vapor deposition (CVD), including lowpressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapordeposition (PVD), atomic layer deposition (ALD), or other suitableprocess. The mask layer 16 is patterned into a mask pattern by usingpatterning operations including photo-lithography and etching.

Next, as shown in FIG. 4, the stacked layers of the first and secondsemiconductor layers 20, 25 are patterned by using the patterned masklayer 16, thereby the stacked layers are formed into fin structures 29extending in the X direction. In FIG. 4, two fin structures 29 arearranged in the Y direction. But the number of the fin structures is notlimited to two, and may be as small as one and three or more. In someembodiments, one or more dummy fin structures are formed on both sidesof the fin structures 29 to improve pattern fidelity in the patterningoperations. As shown in FIG. 4 the fin structures 29 have upper portionsconstituted by the stacked semiconductor layers 20, 25 and well portions11.

The width W1 of the upper portion of the fin structure along the Ydirection is in a range from about 10 nm to about 40 nm in someembodiments, and is in a range from about 20 nm to about 30 nm in otherembodiments. The height H1 along the Z direction of the fin structure isin a range from about 100 nm to about 200 nm.

The stacked fin structure 29 may be patterned by any suitable method.For example, the structures may be patterned using one or morephotolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the stacked finstructure 29.

After the fin structures 29 are formed, an insulating material layerincluding one or more layers of insulating material is formed over thesubstrate so that the fin structures are fully embedded in theinsulating layer. The insulating material for the insulating layer mayinclude silicon oxide, silicon nitride, silicon oxynitride (SiON),SiOCN, SiCN, fluorine-doped silicate glass (FSG), or a low-k dielectricmaterial, formed by LPCVD (low pressure chemical vapor deposition),plasma-CVD or flowable CVD. An anneal operation may be performed afterthe formation of the insulating layer. Then, a planarization operation,such as a chemical mechanical polishing (CMP) method and/or an etch-backmethod, is performed such that the upper surface of the uppermost secondsemiconductor layer 25 is exposed from the insulating material layer. Insome embodiments, a fin liner layer 13 is formed over the fin structuresbefore forming the insulating material layer. The fin liner layer 13 ismade of SiN or a silicon nitride-based material (e.g., SiON, SiCN orSiOCN).

In some embodiments, the fin liner layers 13 include a first fin linerlayer formed over the substrate 10 and sidewalls of the bottom part ofthe fin structures 11, and a second fin liner layer formed on the firstfin liner layer. Each of the liner layers has a thickness between about1 nm and about 20 nm in some embodiments. In some embodiments, the firstfin liner layer includes silicon oxide and has a thickness between about0.5 nm and about 5 nm, and the second fin liner layer includes siliconnitride and has a thickness between about 0.5 nm and about 5 nm. Theliner layers may be deposited through one or more processes such asphysical vapor deposition (PVD), chemical vapor deposition (CVD), oratomic layer deposition (ALD), although any acceptable process may beutilized.

Then, as shown in FIG. 5, the insulating material layer is recessed toform an isolation insulating layer 15 so that the upper portions of thefin structures 29 are exposed. With this operation, the fin structures29 are separated from each other by the isolation insulating layer 15,which is also called a shallow trench isolation (STI). The isolationinsulating layer 15 may be made of suitable dielectric materials such assilicon oxide, silicon nitride, silicon oxynitride, fluorine-dopedsilicate glass (FSG), low-k dielectrics such as carbon doped oxides,extremely low-k dielectrics such as porous carbon doped silicon dioxide,a polymer such as polyimide, combinations of these, or the like. In someembodiments, the isolation insulating layer 15 is formed through aprocess such as CVD, flowable CVD (FCVD), or a spin-on-glass process,although any acceptable process may be utilized.

In the embodiment shown in FIG. 5, the insulating material layer 15 isrecessed until the upper portion of the fin structure (well layer) 11 isexposed. In other embodiments, the upper portion of the fin structure 11is not exposed. The first semiconductor layers 20 are sacrificial layerswhich are subsequently partially removed, and the second semiconductorlayers 25 are subsequently formed into semiconductor wires as channellayers of a GAA FET.

After the isolation insulating layer 15 is formed, a sacrificial (dummy)gate structure 38 is formed, as shown in FIG. 6. FIG. 6 illustrates astructure after a sacrificial gate structure 38 is formed over theexposed fin structures 29. The sacrificial gate structure 38 is formedover a portion of the fin structures which is to be a channel region.The sacrificial gate structure 38 defines the channel region of the GAAFET. The sacrificial gate structure 38 includes a sacrificial gatedielectric layer 31 and a sacrificial gate electrode layer 30. Thesacrificial gate dielectric layer 31 includes one or more layers ofinsulating material, such as a silicon oxide-based material. In oneembodiment, silicon oxide formed by CVD is used. The thickness of thesacrificial gate dielectric layer 31 is in a range from about 1 nm toabout 5 nm in some embodiments.

The sacrificial gate structure 38 is formed by first blanket depositingthe sacrificial gate dielectric layer 31 over the fin structures. Asacrificial gate electrode layer is then blanket deposited on thesacrificial gate dielectric layer and over the fin structures, such thatthe fin structures are fully embedded in the sacrificial gate electrodelayer. The sacrificial gate electrode layer includes silicon such aspolycrystalline silicon or amorphous silicon. The thickness of thesacrificial gate electrode layer is in a range from about 100 nm toabout 200 nm in some embodiments. In some embodiments, the sacrificialgate electrode layer is subjected to a planarization operation. Thesacrificial gate dielectric layer and the sacrificial gate electrodelayer are deposited using CVD, including LPCVD and PECVD, PVD, ALD, orother suitable process. Subsequently, a mask layer is formed over thesacrificial gate electrode layer. The mask layer includes a pad SiNlayer 33 and a silicon oxide mask layer 34.

Next, a patterning operation is performed on the mask layer andsacrificial gate electrode layer is patterned into the sacrificial gatestructure 38, as shown in FIG. 6. The sacrificial gate structureincludes the sacrificial gate dielectric layer 31, the sacrificial gateelectrode layer 30 (e.g., poly silicon), the pad SiN layer 33 and thesilicon oxide mask layer 34. By patterning the sacrificial gatestructure, the stacked layers of the first and second semiconductorlayers are partially exposed on opposite sides of the sacrificial gatestructure, thereby defining source/drain (S/D) regions, as shown in FIG.6. In FIG. 6, one sacrificial gate structure is formed, but the numberof the sacrificial gate structures is not limited to one. Two or moresacrificial gate structures are arranged in the X direction in someembodiments. In certain embodiments, one or more dummy sacrificial gatestructures are formed on both sides of the sacrificial gate structuresto improve pattern fidelity.

Further, a cover layer for sidewall spacers 32 is formed over thesacrificial gate structure 38, as shown in FIG. 7. The cover layer 32 isdeposited in a conformal manner so that it is formed to havesubstantially equal thicknesses on vertical surfaces, such as thesidewalls, horizontal surfaces, and the top of the sacrificial gatestructure, respectively. In some embodiments, the cover layer 32 has athickness in a range from about 5 nm to about 20 nm. The cover layer 32includes one or more of SiN, SiON and SiCN or any other suitabledielectric material. The cover layer 32 can be formed by ALD or CVD, orany other suitable method.

In certain embodiments, before the cover layer 32 is formed, anadditional cover layer 47 made of an insulating material is conformallyformed over the exposed fin structures and the sacrificial gatestructure 38. In such a case, the additional cover layer and the coverlayer are made of different materials so that one of them can beselectively etched. The additional cover layer 47 includes a low-kdielectric material, such as SiOC and/or SiOCN or any other suitabledielectric material and can be formed by ALD or CVD, or any othersuitable method.

Although FIGS. 2-7 shows the structure in which one sacrificial gatestructure is disposed over two fin structures, the structure is notlimited to one sacrificial gate structure. In some embodiments, two ormore sacrificial gate structures are disposed over one or more finstructures. Further, two or more of the structure shown in FIG. 7 aredisposed in different areas (e.g., p-type regions and/or n-type regions)on the semiconductor substrate.

FIG. 8 shows a cross sectional view along the X direction after thecover layer (sidewall spacers) 32 is formed. In FIG. 8 and thereafter,the upper portion and the detailed structure of the sacrificial gatestructure 38 are not shown for simplicity. In FIGS. 8-11 and 15-18, onlyone GAA FET is illustrated, but two or more GAA FETs, which respectivelyhave different source/drain configurations as shown in FIGS. 12A-13F and21A-21D, may be fabricated on the same substrate 10.

Next, as shown in FIG. 9, bottom portions of the cover layer 32 areremoved by anisotropic etching, thereby forming sidewall spacers 32. Insome embodiments, an upper portion of an uppermost second semiconductorlayer 25 is slightly etched. In other embodiments, the uppermostsemiconductor layer 25 is not substantially etched.

Subsequently, as shown in FIG. 10, the first semiconductor layers 20 inthe source/drain region are removed by using one or more lithography andetching operations, thereby forming a first S/D space 21. In someembodiments, the substrate 10 (or the bottom part of the fin structures11) is also partially etched. Further, in some embodiments, the firstsemiconductor layers 20 are laterally etched in the X direction. Theamount of etching of the first semiconductor layer 20 is in a range fromabout 2 nm to about 10 nm in some embodiments. When the firstsemiconductor layers 20 are Ge or SiGe and the second semiconductorlayers 25 are Si, the first semiconductor layers 20 can be selectivelyetched by using a wet etchant such as, but not limited to, ammoniumhydroxide (NH₄OH), tetramethylammonium hydroxide (TMAH), ethylenediaminepyrocatechol (EDP), or potassium hydroxide (KOH) solutions.

Then, as shown in FIG. 11, a dielectric layer 35 is formed on thelateral ends of the first semiconductor layer 20 and on the secondsemiconductor layer 25 in the first S/D space 21. The dielectric layer35 is made of suitable dielectric materials such as silicon oxide,silicon oxynitride, fluorine-doped silicate glass (FSG), low-kdielectrics such as carbon doped oxides (SiOC, SiOCN), extremely low-kdielectrics such as porous carbon doped silicon dioxide, a polymer suchas polyimide, combinations of these, or the like. In some embodiments,the dielectric layer 35 includes one or more layers of low-k dielectricmaterial. The dielectric layer 35 is formed so as to fully fill thefirst S/D space 21 and also formed on sidewall spacers 32, in someembodiments. The dielectric layer 35 can be formed through a processsuch as CVD, flowable CVD (FCVD), ALD, or a spin-on-glass process,although any acceptable process may be utilized.

In some embodiments, before forming the dielectric layer 35, aninsulating layer is conformally formed on the lateral ends of the firstsemiconductor layer 20 and on the second semiconductor layer 25. Theinsulating layer functions as an etch-stop layer in the subsequentchannel formation operations. The insulating layer includes one ofsilicon nitride (SiN) and silicon oxide (SiO₂), and has a thickness in arange from about 0.5 nm to about 3.0 nm. In other embodiments, theinsulating layer has a thickness in a range from about 1.0 nm to about2.0 nm. The insulating layer can be formed by ALD or any other suitablemethod.

After the dielectric layer 35 is formed, the source/drain region isetched to form a second S/D space 22, as shown in FIGS. 12A-12D.Depending on the desired drive current (or resistance), the depth of thesecond S/D space 22 is adjusted. For example, in the case of FIG. 12A,the dielectric layer 35 and only the uppermost second semiconductorlayer 25 are etched, while the remaining second semiconductor layers 25remain embedded in the dielectric layer 35. In the case of FIG. 12B, twoupper second semiconductor layers 25 and the dielectric layer 35 areetched to expose end portions of the two upper second semiconductorlayers 25 in the second S/D space 22. Similarly, in the case of FIG.12C, three second semiconductor layers 25 from the top and thedielectric layer 35 are etched to expose end portions of the threesecond semiconductor layers 25 in the second S/D space 22. In the caseof FIG. 12D, the dielectric layer 35 and the second semiconductor layers25 are etched so that end portions of all of the second semiconductorlayers 25 are exposed in the second S/D space 22. In some embodiments,all of the structures shown in FIGS. 12A-12D are provided on the samesubstrate 10 (on one semiconductor chip). In other embodiments, onlysome of the structures shown in FIGS. 12A-12D are provided on the samesubstrate 10. The structures of FIGS. 12A-12D can be formed by one ormore lithography and etching operations.

FIGS. 13A-13F show the structures after the second S/D spaces 22 areformed in devices having vertically arranged six second semiconductorlayers 25. In some embodiments, all of the structures shown in FIGS.13A-13F are provided on the same substrate 10, and in other embodiments,only some of the structures shown in FIGS. 13A-13F are provided on thesame substrate 10. The structures of FIGS. 12A-12D can be formed by oneor more lithography and etching operations.

FIG. 14 shows the one or more lithography and etching operations tofabricate the structures shown in FIGS. 13A-13F on the same substrate,according to some embodiments. To fabricate all of the structures shownin FIGS. 13A-13F, at most three lithography/etching operations can beperformed. For example, the first, third and fifth S/D regions for aone-wire contact structure, a three-wire contact structure and afive-wire contact structure, respectively, are subjected to a firstetching operation to a depth of D, which corresponds to an etching depthto cut and expose ends of the uppermost second semiconductor layer 25,but not reach the next second semiconductor layer 25. The second, fourthand sixth S/D regions for a two-wire contact structure, a four-wirecontact structure and a six-wire contact structure, respectively, arecovered by, for example, photoresist formed by a lithography operation.Then, the second, third and sixth S/D regions are subjected to a secondetching operation to a depth of 2D, while the first, fourth and fifthS/D regions are covered. Further, the fourth, fifth and sixth S/Dregions are subjected to a third etching operation to a depth of 3D,while the first, second and third S/D regions are covered. The order ofthe first to third etching operations can be any order.

The number of lithography/etching operations varies depending on thenumber of second semiconductor layers 25. When the number of the secondsemiconductor layer 25 is 1 to 3, the number of lithography/etchingoperations can be 2 (depth D and 2D etching operations), when the numberof the second semiconductor layer 25 is 4 to 7, the number oflithography/etching operations can be 3 (depth D, 2D and 4D etchingoperations), and when the number of the second semiconductor layer 25 is8 to 13, the number of lithography/etching operations can be 4 (depth D,2D, 4D and 6D etching operations).

After the second S/D space 22 is formed, a source/drain epitaxial layer40 is formed, as shown in FIG. 15. In the following figures, themanufacturing operations after the structure shown in FIG. 12C(three-wire contact case) is formed are explained. However, the sameoperations can be applied to the structures shown in FIGS. 12A, 12B, 12Dand 13A-13F.

The source/drain epitaxial layer 40 includes one or more layers of Si,SiP, SiC and SiCP for an n-channel FET or Si, SiGe, Ge for a p-channelFET. For the P-channel FET, boron (B) may also be contained in thesource/drain. The source/drain epitaxial layer 40 is formed by anepitaxial growth method using CVD, ALD or molecular beam epitaxy (MBE).As shown in FIG. 15, the source/drain epitaxial layer 40 is formed incontact with the second semiconductor layers 25.

Then, as shown in FIG. 16, an interlayer dielectric (ILD) layer 36 isformed over the S/D epitaxial layer 40. The materials for the ILD layer36 include compounds comprising Si, 0, C and/or H, such as siliconoxide, SiCOH and SiOC. Organic materials, such as polymers, may be usedfor the ILD layer 36. After the ILD layer 36 is formed, a planarizationoperation, such as CMP, is performed, so that the top portion of thesacrificial gate electrode layer 30 is exposed.

Then, the sacrificial gate structure 38 including the sacrificialelectrode layer 30 and sacrificial gate dielectric layer 31 are removed,thereby forming a gate space 39, as shown in FIG. 17. The ILD layer 36protects the S/D epitaxial layer 40 during the removal of thesacrificial gate structures. The sacrificial gate structures can beremoved using plasma dry etching and/or wet etching. When thesacrificial gate electrode layer 30 is polysilicon and the ILD layer 36is silicon oxide, a wet etchant such as a TMAH solution can be used toselectively remove the sacrificial gate electrode layer 30. Thesacrificial gate dielectric layer 31 is thereafter removed using plasmadry etching and/or wet etching.

After the sacrificial gate structures are removed, the firstsemiconductor layers 20 are removed, thereby forming wires of the secondsemiconductor layers 25, as channel regions, as shown in FIG. 18. Thefirst semiconductor layers 20 can be removed or etched using an etchantthat can selectively etch the first semiconductor layers 20 against thesecond semiconductor layers 25, as set forth above. In some embodiments,when the insulating layer is formed before the dielectric layer 35 isformed, the etching of the first semiconductor layers 20 stops at theinsulating layer.

After the semiconductor wires of the second semiconductor layers 25 areformed, a gate dielectric layer 53 is formed around each channel layers(wires of the second semiconductor layers 25), and a gate electrodelayer 58 is formed on the gate dielectric layer 53, as shown in FIG. 19.

In some embodiments, the gate dielectric layer 53 includes aninterfacial layer 52 and a high-k dielectric layer 54. The interfaciallayer 52 is a chemically formed silicon oxide in some embodiments. Incertain embodiments, the high-k gate dielectric layer 54 includes one ormore layers of a dielectric material, such as HfO₂, HfSiO, HfSiON,HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide, titanium oxide,hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable high-kdielectric materials, and/or combinations thereof.

The high-k gate dielectric layer 54 can be formed by CVD, ALD or anysuitable method. In one embodiment, the high-k gate dielectric layer 54is formed using a highly conformal deposition process such as ALD inorder to ensure the formation of a gate dielectric layer having auniform thickness around each channel layers. The thickness of thehigh-k gate dielectric layer 54 is in a range from about 1 nm to about 6nm in one embodiment.

The gate electrode layer 58 is formed on the gate dielectric layer 53 tosurround each channel layer. The gate electrode layer 58 includes one ormore layers of conductive material, such as polysilicon, aluminum,copper, titanium, tantalum, tungsten, cobalt, molybdenum, tantalumnitride, nickel silicide, cobalt silicide, TiN, WN, TiAl, TiAlN, TaCN,TaC, TaSiN, metal alloys, other suitable materials, and/or combinationsthereof.

The gate electrode layer 58 may be formed by CVD, ALD, electro-plating,or other suitable method. The gate electrode layer is also depositedover the upper surface of the ILD layer 36. The gate dielectric layerand the gate electrode layer formed over the ILD layer 46 are thenplanarized by using, for example, CMP, until the top surface of the ILDlayer 36 is revealed. In some embodiments, after the planarizationoperation, the gate electrode layer 58 is recessed and a cap insulatinglayer (not shown) is formed over the recessed gate electrode layer 58.The cap insulating layer includes one or more layers of a siliconnitride-based material, such as SiN. The cap insulating layer can beformed by depositing an insulating material followed by a planarizationoperation.

In certain embodiments, one or more work function adjustment layers 56are interposed between the gate dielectric layer 53 and the gateelectrode layer 58. The work function adjustment layers 56 are made of aconductive material such as a single layer of TiN, TaN, TaAlC, TiC, TaC,Co, Al, TiAl, HfTi, TiSi, TaSi or TiAlC, or a multilayer of two or moreof these materials. For the n-channel FET, one or more of TaN, TaAlC,TiN, TiC, Co, TiAl, HfTi, TiSi and TaSi is used as the work functionadjustment layer, and for the p-channel FET, one or more of TiAlC, Al,TiAl, TaN, TaAlC, TiN, TiC and Co is used as the work functionadjustment layer. The work function adjustment layer 56 may be formed byALD, PVD, CVD, e-beam evaporation, or other suitable process. Further,the work function adjustment layer 56 may be formed separately for then-channel FET and the p-channel FET which may use different metallayers.

Subsequently, contact holes 37 are formed in the ILD layer 36 by usingdry etching, thereby exposing the upper portion of the S/D epitaxiallayer 40, as shown in FIG. 20. In some embodiments, a silicide layer isformed over the S/D epitaxial layer 40. The silicide layer includes oneor more of WSi, CoSi, NiSi, TiSi, MoSi and TaSi. Then, a conductivecontact layer 60 is formed in the contact holes, and then a conductivecontact plug 65 is formed on the conductive contact layer 60, as shownin FIGS. 21A-21D. FIGS. 21A, 21B, 21C and 21D correspond to FIGS. 12D,12C, 12B and 12A, respectively.

The conductive contact layer 60 includes one or more of Co, Ni, W, Ti,Ta, Cu, Al, TiN and TaN formed by ALD, PVD, CVD, e-beam evaporation, orother suitable process. The conductive contact plug 65 includes one ormore layers of Co, Ni, W, Ti, Ta, Cu, Al, TiN and TaN formed by ALD,PVD, CVD, e-beam evaporation, or other suitable process.

It is understood that the GAA FETs undergoes further CMOS processes toform various features such as contacts/vias, interconnect metal layers,dielectric layers, passivation layers, etc.

FIG. 22 shows a cross sectional view of a semiconductor FET deviceaccording to another embodiment of the present disclosure. Material,configuration, dimensions and/or processes the same as or similar to theforegoing embodiments described with respect to FIGS. 1-21D may beemployed in the embodiment of FIG. 22, and detailed explanation thereofmay be omitted.

In the embodiment of FIG. 22, the first semiconductor layers 20 areutilized as channel regions (semiconductor wires). In one embodiment,the structure of FIG. 22 is a p-type GAA FET. In some embodiments, thefirst semiconductor layers 20 are made of Si_(1-x)Ge_(x), where 0<x≤1.0.In certain embodiments, 0.3≤x≤0.8. The source/drain epitaxial layer 41includes one or more layers of Si, SiGe and Ge. Although FIG. 22 showsthe structure of a two-wire contact case, the number of semiconductorwires 20 connected to the source/drain epitaxial layer 41 is not limitedto two. Where the first semiconductor layers 20 are utilized as channelregions, substantially the same manufacturing operations as set forthabove can be applied.

FIGS. 23-25F show various stages of manufacturing a semiconductor FETdevice according to another embodiment of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIGS. 23-25F, and some of the operationsdescribed below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIGS. 1-22 may be employed in the embodiment of FIGS.23A-25F, and detailed explanation thereof may be omitted.

In the foregoing embodiments, when the second S/D space 22 is formed,one or more of the second semiconductor layers 25 are etched togetherwith the dielectric layer 35. In this embodiment, the dielectric layer35 is selectively etched, thereby exposing one or more of the secondsemiconductor layer 25 to cross the second S/D space 23, as show in FIG.23. In FIG. 23, the four-wire contact structure and two-wire contactstructure are formed on the same substrate 10. However, the embodimentis not limited to this configuration. Then, similar or the sameoperations explained with reference to FIGS. 15-21D are performed, andthe structure shown in FIG. 24 can be obtained. FIGS. 25A-25D show thecase that six second semiconductor layers 25 are employed.

FIGS. 26 and 27 show applications of the GAA FETs with different drivingcurrent capacity. Material, configuration, dimensions and/or processesthe same as or similar to the foregoing embodiments described withrespect to FIGS. 1-25 may be employed in the embodiment of FIGS. 26 and27, and detailed explanation thereof may be omitted.

FIG. 26 shows a NOR circuit and FIG. 27 shows a NAND circuit. In the NORcircuit shown in FIG. 26, transistors M3 and M4 (e.g., p-typetransistors) are connected in series, while transistors M1 and M2 (e.g.,n-type transistors) are connected in parallel, between Vdd and Vss(ground). Thus, the transistors M3 and M4 require larger currentcapacity than the transistors M1 and M2. For this reasons, in someembodiments, the transistor M3 and M4 employ GAA FETs with foursemiconductor wires being connected to the source/drain epitaxial layer,while the transistor M1 and M2 employ GAA FETs with two semiconductorwires connected to the source/drain epitaxial layer. The number ofsemiconductor wires connected to the source/drain epitaxial layer can beadjusted based on device requirements.

In contrast, in the NAND circuit shown in FIG. 27, transistors M3 and M4(e.g., p-type transistors) are connected in parallel, while transistorsM1 and M2 (e.g., n-type transistors) are connected in series, betweenVdd and Vss (ground). Thus, the transistors M1 and M2 require largercurrent capacity than the transistors M3 and M4. For this reason, insome embodiments, the transistors M1 and M1 employ GAA FETs with foursemiconductor wires being connected to the source/drain epitaxial layer,while the transistors M3 and M4 employ GAA FETs with two semiconductorwires being connected to the source/drain epitaxial layer. The number ofsemiconductor wires connected to the source/drain epitaxial layer can beadjusted based on device requirements.

As shown in FIGS. 26 and 27, in the present disclosure, the drivingcurrent capacity of a GAA FET can easily be adjusted by adjusting thenumber of semiconductor wires connected to the source/drain epitaxiallayer, which can be achieved by adjusting an etching depth of the secondS/D space.

FIGS. 28A-29B show another application of the GAA FETs with differentdriving current capacity. Material, configuration, dimensions and/orprocesses the same as or similar to the foregoing embodiments describedwith respect to FIGS. 1-27 may be employed in the embodiment of FIGS.28A and 28B, and detailed explanation thereof may be omitted.

FIG. 28B shows a SRAM unit cell layout corresponding to a circuitdiagram shown in FIG. 20A, and FIG. 28A corresponds to the cross sectioncutting transistors TR3 and TR6 of FIG. 28B.

In FIG. 28A, two GAA FETs TR3 and TR6 share one drain region including adrain epitaxial layer 40B. In the SRAM unit cell, the transistor TR3 isa pull-down transistor and transistor TR6 is a pass-gate transistor,which are both, for example, n-type transistors.

In this embodiment, the numbers of active semiconductor wires 25(channel regions) of the GAA FET TR3 and that of GAA FET TR6 isdifferent. In some embodiments, the GAA FET TR3 includes three activesemiconductor wires 25, while the GAA FET TR6 includes four activesemiconductor wires 25. Similar or the same structure is applied totransistors TR1 and TR5, which are another pull-down transistor andpass-gate transistor. As shown in FIG. 28A, source structures areasymmetric with respect to the common drain. The number of active wiresof pull-up transistor TR2 and TR4 (p-type transistor) may be the same asor different from that of the pull-down transistors TR1 and TR3 and/orthe pass-gate transistors TR5 and TR6.

In a SRAM device, pull-down transistors generally require a drivingcurrent capacity equal to or higher than pass-gate transistors andrequire a driving current capacity higher than pull-up transistors, andthe pass-gate transistors generally require a driving current capacityequal to or higher than pull-up transistors. FIGS. 29A and 29B show someconfigurations of ratios of the driving current capacities for thesetransistors in a SRAM device. Although FIG. 29B shows various ratios ofdriving current capacities, it can also be read to show numbers (orratios in numbers) of active semiconductor wires connected to thesource/drain epitaxial layer.

A signal-to-noise margin (Read/Hold) can be improved when the PD/PGratio in driving capacities is higher and the driving capacity ofpull-up transistors is higher. A write margin can be improved when thePG/PU ratio in driving capacities is higher. By adjusting the number ofactive semiconductor wires connected to the source/drain epitaxiallayer, the driving current capacities of GAA FETs in the SRAM cell canbe easily adjusted. In addition, reducing the number of activesemiconductor wires connected to the source/drain epitaxial layer in aSRAM cell can also reduce an input capacitance and further improve thepower and speed of the circuit. By using the GAA FETs with the presentembodiments, it is possible to reduce an SRAM cell area by about 6%compared with an SRAM cell using Fin FETs with multiple fins.

The various embodiments or examples described herein offer severaladvantages over the existing art. For example, in the presentdisclosure, by adjusting the number of active semiconductor wiresconnected to the source/drain epitaxial layer, the driving currentcapacities of GAA FETs and the input capacitance can easily be adjusted.Further, it is possible to minimize the number of fabrication operationsand reduce the size of a semiconductor device.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

In accordance with an aspect of the present disclosure, in a method ofmanufacturing a semiconductor device, a fin structure, in which firstsemiconductor layers and second semiconductor layers are alternatelystacked, is formed. A sacrificial gate structure is formed over the finstructure. The first semiconductor layers are etched at a source/drainregion of the fin structure, which is not covered by the sacrificialgate structure, thereby forming a first source/drain space in which thesecond semiconductor layers are exposed. A dielectric layer is formed atthe first source/drain space, thereby covering the exposed secondsemiconductor layers. The dielectric layer and part of the secondsemiconductor layers are etched, thereby forming a second source/drainspace. A source/drain epitaxial layer is formed in the secondsource/drain space. At least one of the second semiconductor layers isin contact with the source/drain epitaxial layer, and at least one ofthe second semiconductor layers is separated from the source/drainepitaxial layer disposed thereabove by the dielectric layer. In one ormore of the foregoing or following embodiments, the dielectric layerincludes a low-k dielectric material. In one or more of the foregoing orfollowing embodiments, after the source/drain epitaxial layer is formed,the sacrificial gate structure is removed, thereby exposing a part ofthe fin structure, the first semiconductor layers are removed from theexposed fin structure, thereby forming channel layers including thesecond semiconductor layers, and a gate dielectric layer and a gateelectrode layer are formed around the channel layers. In one or more ofthe foregoing or following embodiments, the gate electrode layer wrapsaround the at least one of the second semiconductor layers separatedfrom the source/drain epitaxial layer. In one or more of the foregoingor following embodiments, the at least one of the second semiconductorlayers separated from the source/drain epitaxial layer is located closerto a substrate than remaining second semiconductor layer contacting thesource/drain epitaxial layer. In one or more of the foregoing orfollowing embodiments, two or more of the second semiconductor layersare separated from the source/drain epitaxial layer. In one or more ofthe foregoing or following embodiments, only one of the secondsemiconductor layers is in contact with the source/drain epitaxiallayer. In one or more of the foregoing or following embodiments, thefirst semiconductor layers are made of SiGe, and the secondsemiconductor layers are made of Si.

According to another aspect of the present disclosure, in a method ofmanufacturing a semiconductor device, a fin structure, in which firstsemiconductor layers and second semiconductor layers are alternatelystacked, is formed. A sacrificial gate structure is formed over the finstructure. The first semiconductor layers are etched at a source/drainregion of the fin structure, which is not covered by the sacrificialgate structure, thereby forming a first source/drain space in which thesecond semiconductor layers are exposed. A dielectric layer is formed atthe source/drain region, thereby covering the exposed secondsemiconductor layers. The dielectric layer is etched, thereby forming asecond source/drain space, at least one of the second semiconductorlayers being exposed in and crossing the second source/drain space. Asource/drain epitaxial layer is formed in the second source/drain space.At least one of the second semiconductor layers exposed in the secondsource/drain space is in contact with the source/drain epitaxial layer,and at least one of the second semiconductor layers is separated fromthe source/drain epitaxial layer disposed thereabove by the dielectriclayer. In one or more of the foregoing or following embodiments, thedielectric layer includes a low-k dielectric material. In one or more ofthe foregoing or following embodiments, after the source/drain epitaxiallayer is formed, the sacrificial gate structure is removed, therebyexposing a part of the fin structure, the first semiconductor layers areremoved from the exposed fin structure, thereby forming channel layersincluding the second semiconductor layers, and a gate dielectric layerand a gate electrode layer are formed around the channel layers. In oneor more of the foregoing or following embodiments, the gate electrodelayer wraps around the at least one of the second semiconductor layersseparated from the source/drain epitaxial layer. In one or more of theforegoing or following embodiments, the at least one of the secondsemiconductor layers separated from the source/drain epitaxial layer islocated closer to a substrate than remaining second semiconductor layercontacting the source/drain epitaxial layer. In one or more of theforegoing or following embodiments, two or more of the secondsemiconductor layers are separated from the source/drain epitaxiallayer. In one or more of the foregoing or following embodiments, onlyone of the second semiconductor layers is in contact with thesource/drain epitaxial layer. In one or more of the foregoing orfollowing embodiments, the first semiconductor layers are made of SiGe,and the second semiconductor layers are made of Si.

In accordance with another aspect of the present application, in amethod of manufacturing a semiconductor device, a first fin structureand a second fin structure, in both of which first semiconductor layersand second semiconductor layers are alternately stacked, are formed. Afirst sacrificial gate structure is formed over the first fin structure,and a second sacrificial gate structure is formed over the second finstructure. The first semiconductor layers are removed from a firstsource/drain region of the first fin structure, which is not covered bythe first sacrificial gate structure, and the first semiconductor layersare removed from a second source/drain region of the second finstructure, which is not covered by the second sacrificial gatestructure. A first insulating layer is formed around the secondsemiconductor layers in the first and second source/drain regions. Thedielectric layer and part of the second semiconductor layers are etchedat the first source/drain region, thereby forming a first source/drainspace. The dielectric layer and part of the second semiconductor layersare etched at the second source/drain region, thereby forming a secondsource/drain space. A first source/drain epitaxial layer is formed inthe first source/drain space, and a second source/drain epitaxial layeris formed in the second source/drain space. A number of the secondsemiconductor layers contacting the first source/drain epitaxial layerin the first source/drain region is different from a number of thesecond semiconductor layers contacting the second source/drain epitaxiallayer in the second source/drain region. In one or more of the foregoingor following embodiments, the dielectric layer includes a low-kdielectric material. In one or more of the foregoing or followingembodiments, the first semiconductor layers are made of SiGe, and thesecond semiconductor layers are made of Si. In one or more of theforegoing or following embodiments, a total number of the secondsemiconductor layers in the first fin structure is equal to a totalnumber of the second semiconductor layers in the first fin structure,and the total number of the second semiconductor layers is in a rangefrom 3 to 15.

In accordance with another aspect of the present disclosure, in a methodof manufacturing a semiconductor device, a fin structure, in which firstsemiconductor layers and second semiconductor layers are alternatelystacked, is formed. A sacrificial gate structure is formed over the finstructure. The first semiconductor layers are etched at a source regionand a drain region of the fin structure, which are not covered by thesacrificial gate structure, thereby forming a first source space and afirst drain space, in both of which the second semiconductor layers areexposed. A dielectric layer is formed at the first source space and thefirst drain space, thereby covering the exposed second semiconductorlayers. The dielectric layer and part of the second semiconductor layersare etched, thereby forming a second source space and a second drainspace. A source epitaxial layer is formed in the second source space anda drain epitaxial layer is formed in the second drain space. A number ofthe second semiconductor layers contacting the source epitaxial layer inthe source region is different from a number of the second semiconductorlayers contacting the drain epitaxial layer in the drain region.

In accordance with one aspect of the present disclosure, a semiconductordevice includes a first gate-all-around field effect transistor (GAAFET) disposed over a substrate, and a second GAA FET disposed over thesubstrate. Each of the first GAA FET and the second GAA FET includessemiconductor wires vertically arranged over the substrate, asource/drain epitaxial layer in contact with one or more of thesemiconductor wires, a gate dielectric layer disposed on and wrappingaround each channel region of the semiconductor wires, and a gateelectrode layer disposed on the gate dielectric layer and wrappingaround the each channel region. In at least one of the first GAA FET andthe second GAA FET, at least one of the semiconductor wires is separatedfrom the source/drain epitaxial layer disposed thereabove by adielectric layer. In one or more of the foregoing or followingembodiments, a number of the semiconductor wires contacting thesource/drain epitaxial layer in the first GAA FET is different from anumber of the semiconductor wires contacting the source/drain epitaxiallayer in the second GAA FET. In one or more of the foregoing orfollowing embodiments, the dielectric layer includes a low-k dielectricmaterial. In one or more of the foregoing or following embodiments, thenumber of the semiconductor wires contacting the source/drain epitaxiallayer in the first GAA FET is greater than the number of thesemiconductor wires contacting the source/drain epitaxial layer in thesecond GAA FET, and in the second GAA FET, at least one of thesemiconductor wires is separated from the source/drain epitaxial layerdisposed thereabove by a dielectric layer. In one or more of theforegoing or following embodiments, the gate electrode layer wrapsaround the at least one of the second semiconductor wires separated fromthe source/drain epitaxial layer in the second GAA FET. In one or moreof the foregoing or following embodiments, the at least one of thesemiconductor wires separated from the source/drain epitaxial layer islocated closer to the substrate than remaining one or more semiconductorwires contacting the source/drain epitaxial layer. In one or more of theforegoing or following embodiments, in the second GAA FET, two or moreof the semiconductor wires are separated from the source/drain epitaxiallayer. In one or more of the foregoing or following embodiments, in thesecond GAA FET, only one of the semiconductor wires is in contact withthe source/drain epitaxial layer. In one or more of the foregoing orfollowing embodiments, in the first GAA FET, at least one of thesemiconductor wires is separated from the source/drain epitaxial layerdisposed thereabove by the dielectric layer. In one or more of theforegoing or following embodiments, in the first GAA FET, all of thesemiconductor wires are in contact with the source/drain epitaxiallayer. In one or more of the foregoing or following embodiments, anumber of the semiconductor wires contacting the source/drain epitaxiallayer in the first GAA FET is equal to a number of the semiconductorwires contacting the source/drain epitaxial layer in the second GAA FET.

In accordance with another aspect of the present disclosure, asemiconductor device includes semiconductor wires vertically arrangedover a substrate, a source/drain epitaxial layer in contact with one ormore of the semiconductor wires, a gate dielectric layer disposed on andwrapping around each channel region of the semiconductor wires, and agate electrode layer disposed on the gate dielectric layer and wrappingaround the each channel region. At least one of the semiconductor wiresis separated from the source/drain epitaxial layer disposed thereaboveby a dielectric layer. In one or more of the foregoing or followingembodiments, the gate electrode layer wraps around the at least one ofthe second semiconductor wires separated from the source/drain epitaxiallayer in the second GAA FET. In one or more of the foregoing orfollowing embodiments, two or more of the semiconductor wires areseparated from the source/drain epitaxial layer by the dielectric layer.

In accordance with another aspect of the present disclosure, asemiconductor device includes a gate-all-around field effect transistor(GAA FET). The GAA FET includes semiconductor wires vertically arrangedover a substrate, a source epitaxial layer in contact with one or moreof the semiconductor wires, a drain epitaxial layer in contact with oneor more of the semiconductor wires, a gate dielectric layer disposed onand wrapping around each channel region of the semiconductor wires, anda gate electrode layer disposed on the gate dielectric layer andwrapping around the each channel region. A number of the semiconductorwires contacting the source epitaxial layer is different from a numberof the semiconductor wires contacting the drain epitaxial layer. In oneor more of the foregoing or following embodiments, at least one of thesemiconductor wires is separated from at least one of the source anddrain epitaxial layers by a dielectric layer. In one or more of theforegoing or following embodiments, the gate electrode layer wrapsaround the at least one of the second semiconductor wires separated fromthe at least one of the source and drain epitaxial layers. In one ormore of the foregoing or following embodiments, all of the semiconductorwires are in contact with the source epitaxial layer. In one or more ofthe foregoing or following embodiments, at least one of thesemiconductor wires is separated from the source epitaxial layer by adielectric layer, and two or more of the semiconductor wires areseparated from the source epitaxial layer. In one or more of theforegoing or following embodiments, only one of the semiconductor wiresis in contact with the source epitaxial layer.

In accordance with another aspect of the present disclosure, asemiconductor device includes semiconductor wires vertically arrangedover a substrate, a source/drain epitaxial layer wrapping around eachsource/drain region of one or more of the semiconductor wires, a gatedielectric layer disposed on and wrapping around each channel region ofthe semiconductor wires, and a gate electrode layer disposed on the gatedielectric layer and wrapping around the each channel region. At leastone of the semiconductor wires is separated from the source/drainepitaxial layer disposed thereabove by a dielectric layer.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising: forming a fin structure in which first semiconductor layersand second semiconductor layers are alternately stacked; forming asacrificial gate structure over the fin structure; etching the firstsemiconductor layers at a source/drain region of the fin structure,which is not covered by the sacrificial gate structure, thereby forminga first source/drain space in which the second semiconductor layers areexposed; forming a dielectric layer at the first source/drain space,thereby covering the exposed second semiconductor layers; etching thedielectric layer and part of the second semiconductor layers, therebyforming a second source/drain space; and forming a source/drainepitaxial layer in the second source/drain space, wherein: at least oneof the second semiconductor layers is in contact with the source/drainepitaxial layer, and at least one of the second semiconductor layers isseparated from the source/drain epitaxial layer disposed thereabove bythe dielectric layer.
 2. The method of claim 1, wherein the dielectriclayer includes a low-k dielectric material.
 3. The method of claim 1,further comprising, after the source/drain epitaxial layer is formed:removing the sacrificial gate structure, thereby exposing a part of thefin structure; removing the first semiconductor layers from the exposedfin structure, thereby forming channel layers including the secondsemiconductor layers; and forming a gate dielectric layer and a gateelectrode layer around the channel layers.
 4. The method of claim 3,wherein the gate electrode layer wraps around the at least one of thesecond semiconductor layers separated from the source/drain epitaxiallayer.
 5. The method of claim 1, wherein the at least one of the secondsemiconductor layers separated from the source/drain epitaxial layer islocated closer to a substrate than remaining second semiconductor layercontacting the source/drain epitaxial layer.
 6. The method of claim 1,wherein two or more of the second semiconductor layers are separatedfrom the source/drain epitaxial layer.
 7. The method of claim 6, whereinonly one of the second semiconductor layers is in contact with thesource/drain epitaxial layer.
 8. The method of claim 1, wherein: thefirst semiconductor layers are made of SiGe, and the secondsemiconductor layers are made of Si.
 9. A method of manufacturing asemiconductor device, comprising: forming a fin structure in which firstsemiconductor layers and second semiconductor layers are alternatelystacked; forming a sacrificial gate structure over the fin structure;etching the first semiconductor layers at a source/drain region of thefin structure, which is not covered by the sacrificial gate structure,thereby forming a first source/drain space in which the secondsemiconductor layers are exposed; forming a dielectric layer at thesource/drain region, thereby covering the exposed second semiconductorlayers; etching the dielectric layer, thereby forming a secondsource/drain space, at least one of the second semiconductor layersbeing exposed in and crossing the second source/drain space; and forminga source/drain epitaxial layer in the second source/drain space,wherein: the at least one of the second semiconductor layers exposed inthe second source/drain space is in contact with the source/drainepitaxial layer, and at least one of the second semiconductor layers isseparated from the source/drain epitaxial layer disposed thereabove bythe dielectric layer.
 10. The method of claim 9, wherein the dielectriclayer includes a low-k dielectric material.
 11. The method of claim 9,further comprising, after the source/drain epitaxial layer is formed:removing the sacrificial gate structure, thereby exposing a part of thefin structure; removing the first semiconductor layers from the exposedfin structure, thereby forming channel layers including the secondsemiconductor layers; and forming a gate dielectric layer and a gateelectrode layer around the channel layers.
 12. The method of claim 11,wherein the gate electrode layer wraps around the at least one of thesecond semiconductor layers separated from the source/drain epitaxiallayer.
 13. The method of claim 9, wherein the at least one of the secondsemiconductor layers separated from the source/drain epitaxial layer islocated closer to a substrate than remaining second semiconductor layercontacting the source/drain epitaxial layer.
 14. The method of claim 9,wherein two or more of the second semiconductor layers are separatedfrom the source/drain epitaxial layer.
 15. The method of claim 14,wherein only one of the second semiconductor layers is in contact withthe source/drain epitaxial layer.
 16. The method of claim 9, wherein:the first semiconductor layers are made of SiGe, and the secondsemiconductor layers are made of Si.
 17. A semiconductor device,comprising: a first gate-all-around field effect transistor (GAA FET)disposed over a substrate; and a second GAA FET disposed over thesubstrate, wherein: each of the first GAA FET and the second GAA FETincludes: semiconductor wires vertically arranged over the substrate; asource/drain epitaxial layer in contact with one or more of thesemiconductor wires; a gate dielectric layer disposed on and wrappingaround each channel region of the semiconductor wires; and a gateelectrode layer disposed on the gate dielectric layer and wrappingaround the each channel region, and in at least one of the first GAA FETand the second GAA FET, at least one of the semiconductor wires isseparated from the source/drain epitaxial layer disposed thereabove by adielectric layer.
 18. The semiconductor device of claim 17, wherein anumber of the semiconductor wires contacting the source/drain epitaxiallayer in the first GAA FET is different from a number of thesemiconductor wires contacting the source/drain epitaxial layer in thesecond GAA FET.
 19. The semiconductor device of claim 18, wherein thedielectric layer includes a low-k dielectric material.
 20. Thesemiconductor device of claim 18, wherein: the number of thesemiconductor wires contacting the source/drain epitaxial layer in thefirst GAA FET is greater than the number of the semiconductor wirescontacting the source/drain epitaxial layer in the second GAA FET, andin the second GAA FET, at least one of the semiconductor wires isseparated from the source/drain epitaxial layer disposed thereabove by adielectric layer.